Sorry I cannot help but am bumping up this thread and with luck someone who can help will have suggestions for who to contact--(possibly the BTO?).
We're waiting on this and a few more reports but we do know where they come from. They were born in Dunkirk, Cambridgeshire and more information on them can be found here dkpzxw.by3301.livefilestore.com/.../Dunkirk%20avocets%20newsletter%201.pdf We have also recorded H7 and A6 here this Spring.
The more you're out the more you see - https://twitter.com/BoyWonderBirder
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